82 Mr. Mossotti on the Constitution of the 
of M. Bessel, ten years and a quarter would scarcely be suffi- 
cient for the light from one of the stars which we may suppose 
the nearest, viz. 61 Cygni, to reach us. And, if we consider 
the smallest stars visible to the naked eye as those which are 
placed at an intermediate distance between the nearest and 
the most remote or telescopic stars, we may presume with 
Sir J. Herschel, without fear of departing greatly from the 
truth, that the light from many of them takes some thou- 
sand years in reaching us: so that, to avail myself of the ex- 
pressive style of that writer, when we observe those stars, 
when we note their changes, we are reading and writing their 
history of a thousand years ago. Yet distant as they may 
seem, these stars do not mark the limits of the universe ; they 
are only those that constitute our sidereal system. There are 
in the heavens groups of stars and divisible nebula, which, 
according to all appearance, form separate sidereal systems. 
The distances at which these systems are placed must be as 
much greater than those of the common stars, as the distances 
of these are greater than the dimensions of our planetary 
system. Following out the increasing progression of these 
intervals, our imagination fails, and is bewildered in the con- 
ception of such an immensity of space. 
Ihave wished, Gentlemen, to recall to your memory these 
notions of the heavenly distances, to prepare your minds, and 
introduce them to the vast theatre in which occur the phe- 
nomena that are to form the subject of my discourse. I 
do not, however, intend to speak of the remotest sidereal sy- 
stems, which are scarcely discoverable by the most powerful 
telescopes; they are too distant, and as yet we know too little 
of them. My lecture will treat of the constitution of the si- 
dereal system of which our sun forms a part, of the form 
according to which it has been fashioned, and of the me- 
chanical conditions which ensure its stability during the 
lapse of centuries. 
No one who has turned his eye to the heavens, but must 
have dwelt with wonder on that streak of light of an irregular 
whiteness which surrounds the heavenly vault like a belt. 
This belt has received the name of Galaxy, or Milky Way, 
ever since poets described it as produced by Juno’s milk 
which the infant Hercules had let drop from his mouth. An 
ancient philosopher, Metrodorus, believed that the Milky Way 
was the sun’s path, and that the luminary having on its pass- 
age kindled the stars with its heat, that portion of the heavens 
remained of a whitish colour owing to their scattered ashes*. 
* Plutarch. de Placitis, lib. iii, cap. i. ; and Manilius, Astronomicon, lib. i. 
vers. 727. et seq. 
